Optical fibers permit convenient delivery of optical beams in a variety of applications. The flexibility of optical fibers enables access to difficult locations, and fiber optic connectors permit fiber replacement without complex optical system realignments that are especially challenging in installed equipment. Optical fibers are inexpensive, and many different fiber designs are commercially available.
Optical fiber-based beam delivery systems tend to produce output beam variations such an unstable beam size, divergence angle, beam position, or beam power. FIGS. 1A-1C are refractive index profiles of several fiber types that can be used. Step index fibers (FIG. 1A) tend to exhibit relatively small variations in output beam diameter but up to about 20% variation in beam divergence. In addition, step index fibers typically provide 20-30% less output power than that available with a gradient index design such as those of FIGS. 1B-1C. Fibers having a refractive index profile similar to that of FIG. 1C tend to produce higher power output beams that vary in position, divergence, and diameter. For example, beam size variations of 20% are possible.
In many applications, output power from an optical fiber must be focused to a particular diameter at a particular location with a particular beam divergence. Variation in diameter, power, or position can result in unacceptable power losses or power variations. In high power applications, such variability can result in optically induced damage to one or more optical components. For these reasons, improved beam delivery methods and apparatus are needed.